In current wireless communications systems, 5 MHz˜20 MHz radio bandwidths are typically used for up to 100 Mbps peak transmission rate. Much higher peak transmission rate is required for next generation wireless systems. For example, 1 Gbps peak transmission rate is required by ITU-R for IMT-Advanced systems such as the 4th generation (“4G”) mobile communications systems. The current transmission technologies, however, are very difficult to perform 100 bps/Hz transmission spectrum efficiency. In the foreseeable next few years, only up to 15 bps/Hz transmission spectrum efficiency can be anticipated. Therefore, much wider radio bandwidths (i.e., at least 40 MHz) will be necessary for next generation wireless communications systems to achieve 1 Gbps peak transmission rate.
Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multiplexing protocol to perform high transmission rate over frequency selective channel without the disturbance from inter-carrier interference. There are two typical architectures to utilize much wider radio bandwidth for OFDM system. In a traditional OFDM system, a single radio frequency (RF) carrier is used to carry one wideband radio signal, and in an OFDM multi-carrier system, multiple RF carriers are used to carry multiple narrower band radio signals. The multi-carrier operation is also known as carrier aggregation or bandwidth extension. An OFDM multi-carrier system has various advantages as compared to a traditional OFDM system such as lower Peak to Average Power Ratio, easier backward compatibility, and more flexibility. Thus, OFDM multi-carrier wireless systems have become the baseline system architecture in IEEE 802.16m and LTE-Advanced draft standards to fulfill system requirements.
In a multi-carrier environment, there exists some channel relationship among different RF carriers. For example, path-loss and shadowing fading are typically the same for all RF carriers. When RF carriers span over a continuous RF band, its channels are highly correlated. It is thus possible to exploit channel correlation for carrier activation process. In addition, Open Loop Power Control (OLPC) is preferred to provide efficient power management. On the other hand, short-term fading is frequency-selective, and Interference over Thermal (IoT) may also be carrier-dependent due to scheduling and carrier properties. For example, IoT is carrier-specific if each RF carrier is applied in different fractional frequency reuse (FFR) region. As a result, Close Loop Power Control (CLPC) per RF carrier is required for RF carriers conveying on-going data traffic.
In order to minimize power consumption, a mobile station (MS) sometimes enters sleep mode operation, during which the MS conducts pre-negotiated periods of absent time from its serving bas station (BS). When sleep mode operation is active, a series of alternating listening windows followed by sleep windows are provided for the MS. In each listening window, the MS wakes up to receive and transmit data packets. In each sleep window, the serving BS does not transmit any data packets to the MS. Traffic indication is executed by the BS during the MS's listening window, a procedure for the BS to indicate whether any downlink traffic allocation is addressed to the MS. In multi-carrier sleep mode operation, more than one RF carrier may be used to execute the traffic indication procedure. In addition, more than one RF carrier may be awakened for data transmission.
FIG. 1 (prior art) illustrates an example of multi-carrier sleep mode operation using “wakeup all” method. As illustrated in FIG. 1, a primary RF carrier is used to execute the traffic indication procedure. When the MS receives traffic indication message (TRF-IND) via primary carrier, all secondary carriers are awakened at the same time regardless of whether TRF-IND indicates data arrival or not. However, such “wakeup all” method introduces power wasting when there is no data to be received for some of the listening windows. Other solutions have been sought to provide more efficient power management for multi-carrier sleep mode operation. For example, Ericsson has proposed to the LTE-Advanced draft standards that Discontinuous Downlink Reception (DRX) parameters may be configured for one component carrier independently of other component carriers (see R2-092959, Apr. 28, 2009). While this proposal increases scheduling flexibility, it also unnecessarily wakes up the MS for non-periodic services.